Experiments using ahot-stage confocal scanning laser microscope (CSLM) havebeen carried out to observe phase transformations in two steels: Si-killedr esulfurized Fe-0.38w tp ct C-1.43 wt pct Mn and Al-killed Fe-0.20 wt pct C-0.87 wt pct Mn. Austenite formation during continuous heating was investigated on the surface of samples that were etched to reveal the ferrite and pearlite regions. It was found that the austenite precipitated first at the pearlite coloniesa nd subsequently in the ferrite phase. The measured advance rates of the g /pearlite front were roughly twicet hoseo ft he g / a front and both interfaces were found to be curved. The g /pearlite migration ratew as found to be in qualitativeagreement with publishedrate equations for isokinetic austenite formation where diffusion is the rate-limiting step. Austenite decomposition was studied during cooling. Widmanstä tten ferrite laths precipitate as distinct colonies from the existing allotriomorphicf errite phase but then also at MnS precipitates. The electron backscatterdiffraction (EBSD) analysis showed that all of the laths in ap articular colony exhibit similar orientation to onea nother but as lightly different orientation than the parent allotriomorph, supporting as ympathetic nucleation mechanism.T he growth rate of the laths was found to vary widelywithin arange of 1.5 to 11 m m/s. The ferrite formation is finally halted by impingement with other advancing fronts. Ther esults are presented in ap henomologicald iscussion, with some quantitativea nalysis of the transformation kinetics.
Migrating austenite/ferrite interfaces in the ferrite regions of an alloy steel, containing 0.20 wt pct C, 0.87 wt pct Mn, and 0.57 wt pct Cr, with a ferrite/pearlite microstructure have been observed during austenitization by a high-temperature confocal scanning laser microscope in order to determine the mechanisms of transformation. The samples were subjected to isothermal (790°C to 850°C) and nonisothermal (0.5°C to 20°C/s) temperature profiles. The kinetic rates extracted from the observations were compared to models for long-range diffusion-controlled and interface reaction-controlled migration. The transition between the two mechanisms was found to occur at T 0 , which defines the temperature and composition at which a partitionless phase transformation is possible. Occurrence of the partitionless, interface-controlled transformation was confirmed by an analysis of carbon distribution and microstructure before and after a sample was subjected to a particular thermal profile. The mobility of such interfaces was found to be in the range 1.6AE10 )13 to 2AE10 )12 m 4 AEJ )1 AEs )1 , which is consistent with previous studies on interface-controlled migration of the reverse reaction, a to c, during cooling of dilute substitutional iron alloys. The diffusioncontrolled migration, at temperatures below T 0 , was found to occur in two stages: an initial stage, at which the growth rate can be predicted by a semi-infinite diffusion model; and a second stage, at which the growth slows more rapidly, possibly due to the overlap of diffusion fields.
The oxidation kinetics, and the structural evolution of the resulting surface scale, of cast transformationinduced plasticity (TRIP) steel (0.97 wt pct Al and 1.11 wt pct Si) has been investigated in the temperature range of 850 °C to 1250 °C under atmospheres with oxygen partial pressures close to 0.2 atm. Direct visualization using a high-temperature confocal scanning laser microscope (CSLM) showed that at 1050 °C and higher temperatures, a liquid oxide phase formed beneath the surface, penetrating and liquefying the scale that had formed initially. After a period of time, which was dependent on temperature, the liquid became fully crystallized. A microprobe analysis of the steel/scale interface indicated an Al 2 O 3 -SiO 2 -FeO n composition in the liquid oxide. This phase formed a network that penetrated the scale. The rest of the outer scale consisted primarily of Fe 2 O 3 , while Al-Si-rich oxides were observed close to the metal/scale interface. Thermogravimetric analysis indicated a parabolic growth rate below 1000 °C and a linear growth rate at 1000 °C. At higher temperatures, a parabolic rate dominated once again. The scale thickness appears to be limited by the time period during which the liquid oxide could contribute to rapid mass transfer, which resulted in the observed linear oxidation rate. As the upper temperature limit of the linear oxidation region is reached, the liquid oxide becomes enriched with FeO n , decreasing the stability of the liquid phase. This leads to crystallization of solid Fe oxides at the surface or the formation of appreciable amounts of Al-and Si-rich oxides at the interface. These processes block access of the liquid oxide to the steel.
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